In recent years a number of manufacturers have introduced portable and highly compact personal computers. While the technology associated with integrated circuit packaging has advanced to the point that a great deal of processing function can be packaged into a small, portable unit, the display devices which have typically been used with these portable computers have had significant limitations associated therewith.
The most commonly used display technology for these laptop computers has been the liquid crystal display, or LCD. In an LCD the characters are formed by selectively darkening areas of a relatively transparent sheet. In the simplest form of these displays, which are seen daily in electronic watches and clocks, as well as in laptop computers, a flat panel of reflective material is positioned behind this relatively transparent sheet in which the darkened images are formed. Thus, the user is able to perceive these images by seeing the contrast between the light reflected back through the non-darkened areas of the transparent sheet and the darkened areas where light is not reflected.
This simplest form of LCD technology is adequate in many instances, particularly in those environments in which the ambient lighting level is high enough that adequate light is available for transmission through the relatively transparent layer to the reflective surface and back through the relatively transparent layer to the viewer's eyes. This explains the phenomenon which is observed by users of this display technology, that the brighter the ambient light, the easier it is to read the images formed on the display device.
Thus, in environments in which the ambient light is not particularly high, the LCD device becomes difficult to read. This problem involves numerous considerations. The comfort level which needs to be provided to the user relates to how intensive the use of the display is to be. That is, the purchaser of a low cost wrist watch may well be satisfied with not being able to see the digits on the wrist watch as clearly in low light environments, because it involves little added difficulty to look at the watch longer to correctly perceive the relatively few digits being considered. Additionally, since the LCD display on a wrist watch is small, the problem has often been reduced by providing a tiny incandescent light bulb at one edge of the display which is adequate to light the display for quick viewing.
The problem of poor visibility of LCDs in low light environments is a highly significant one for the larger LCDs used in laptop computers. These devices are typically large enough to display in excess of twenty lines of text having eighty characters each. They are often viewed for extended periods of time, so it is not feasible to ignore the additional eye strain. A single, or even a great plurality of incandescent light bulbs along the edges of the display will not provide an even enough light source to adequately and uniformly light the entire display area. Additionally, the electrical power requirement for a large incandescent array would effectively render the computer non-portable.
As a solution to the problem of providing a light source for the large LCD used in laptop computers, the reflective panel behind the relatively transparent layer has been replaced by an electroluminescent panel which is powered by a substantially high voltage alternating current source. One representative model of such an electroluminescent panel requires around 140 volt RMS at a frequency of about 700 Hertz. Higher frequencies than this shorten the life of the electroluminescent panel as do wave shapes which are not substantially sinusoidal.
The IBM PC Convertible is a laptop personal computer which offers an LCD having no supplmental lighting capability. The word "convertible" in its product name is descriptive of the feature that the LCD may be readily detached from the laptop computer when it is desired to interface the computer to a different display, such as a cathode ray tube (CRT), when such a display is available and it is not necessary to rely on the internal batteries of the computer.
In providing an LCD which is backlighted with an electroluminescent panel as an option for such a computer, the problem arises of how to provide the relatively high voltage to the electroluminescent panel. Other laptop computers having liquid crystal displays supplementally lighted by electroluminescent panels have included the high voltage power supplies for these panels in the main, system unit portion of the laptop computer. Since their displays are not detachable there has been no safety problem in providing the high voltage in the system unit and conveying it to the display device through a suitably insulated conductor. In fact, a suitably insulated connector could be provided to allow the safe detachment of the display if such a connector were being designed into the system from the beginning. However, in the case of the IBM PC Convertible, the system unit without the high voltage power supply and the low voltage connector are an existing configuration. It would be highly advantageous to be able to allow users of this product the additional advantages of an LCD having supplemental lighting without the requirement to also purchase a new system unit.
Previous circuits packaged in system units of laptop computers to power electroluminescent panels in LCDs have utilized relatively large magnetic components including a transformer operating in a push-pull, parallel resonant circuit. Such a circuit was not adaptable to packaging within the separable LCD enclosure of the system described above having the detachable display. Attempts to size reduce the prior art circuits through the use of exotic magnetic core materials were unsuccessful in achieving enough size reduction to allow packaging within the detachable LCD enclosure so that the same system unit and connector thereon could be used with an improved display having an electroluminescent panel backlighting.
It would, therefore, be highly advantageous to provide a circuit to power the electroluminescent panel for a backlighted LCD with the circuit being packaged with the display itself, rather than needing to be remotely mounted.